Optical waveguides are used to route optical power between pre-selected paths. Waveguides can be fabricated from a number of different materials including both crystalline and amorphous materials. Some of such materials exhibit an electro-optic effect wherein changes in the index of refraction occur in the presence of an electric field. A waveguide can be fabricated by growing successive crystalline layers of, for example, gallium aluminum arsenide (GaAlAs) and gallium arsenide (GaAs). Other examples of materials that are useful in fabricating waveguides are lithium niobate (LiNbO.sub.3), lithium tantalate (LiTaO.sub.3), zinc oxide (ZnO), and glassy polymers doped with non-linear optical moieties. The ability to alter the index of refraction with an electric field is useful for fabricating integrated optical circuits using optical switches and modulators. Optical waveguides may be used to form photonic chips which can be used in optical recording, telecommunication, computing, and imaging graphics.
Optical waveguides having one input branch and two output branches are known as Y switches. U.S. Pat. No. 3,883,220 (Taylor) discloses a Y switch that branches into two spatially separated optical paths. In the absence of an electric field, light entering the waveguide is equally split between the two paths. When an appropriate electric field is applied across one of the optical paths, the index of refraction of that path is lowered and thereby diverts or switches optical energy from that path to the other path. U.S. Pat. No. 4,070,092 (Burns) discloses a Y switch wherein the index of refraction of one arm of the Y is raised a small amount, .increment.n, by an electric field and the refractive index of the second arm is lowered by that same amount.
Optical waveguides having two input branches and two output branches are known as X switches. U.S. Pat. No. 4,775,207 (Silberberg) discloses an X switch which includes two convergent input and two divergent output waveguides, and electrodes for generating an electric field adjacent to the point of convergence of the output waveguides.
In an X or Y switch where both output branches have the same refractive index, optical energy entering the switch in any given local normal mode will be split equally between the output branch waveguides. However, if one of the output branches has a higher refractive index than the other branch, the lower-order local normal modes will tend to exit through the output branch waveguide that has the higher index of refraction, while the higher-order local normal modes will tend to exit through the output branch waveguide that has the lower refractive index.
A digital switch is one through which light propagates nearly adiabatically. Adiabatic propagation implies a slow enough change in waveguide parameters that optical energy entering the switch in a given local normal mode remains essentially in that mode when passing through the output branch waveguides. Thus there is no substantial mode conversion, or power transfer, between the local normal modes.
Thus, if optical energy enters the switch in the lower-order local normal mode, the light will tend to exit through the output branch waveguide that has the higher refractive index, resulting in a high extinction ratio if the propagation of the energy through the switch is substantially adiabatic. The extinction ratio is the amount of optical energy exiting one branch divided by the amount of optical energy exiting the other branch. Extinction ratios are typically expressed logarithmically in decibels (dB): 10 dB is equivalent to a ratio of 10:1, 20 dB is 100:1, and 30 dB is 1,000:1.
Essentially all of the light which enters the switch exits through one side branch or the other. "Loss" is defined as the percentage of light entering the switch that exits through the "off" branch or radiates away into the background, e.g., the substrate, etc.
Adiabatic propagation will not occur unless the angle between adjacent output branches is small. As discussed by Y. Silberberg, P. Perimutter, and J. E. Baran in their article entitled, "Digital Optical Switch," appearing in Applied Physics Letters 51 (16), Oct. 19, 1987, pp. 1230-1232, the angle should be much smaller than .delta..beta./.gamma., where .delta..gamma. is the average difference between the propagation constants of the two normal modes and .gamma. is their transverse propagation constant in the cladding region. Typically, .gamma..dbd.50.delta..beta., which yields an angle much smaller than 0.02 radians (1.1.degree.).
Silberberg et al. claim an extinction ratio of 20 dB at .+-.15 volts for an angle between side branches of 1 milliradian (0.06.degree. ). Because this angle is so small, it was necessary for Silberberg to make the switch at least 1.5 cm long in order to separate the end of the output branch waveguides far enough (15 .mu.m) to ensure that coupling between the side branches is negligible by the time the light reaches the end of the switch. It is this separation distance of 15 .mu.m between the branches that is critical to uncoupling the light between the branches of the switch. The length of the switch required to uncouple the light is related trigonometrically to the angle between the branches and their separation at the end of the switch. Thus, the switch of Silberberg et al. can be shortened only by increasing the angle between the branches. But as the angle is increased, the propagation becomes increasingly less adiabatic until the switch ceases to act digitally.
The angle between the side branches necessary for digital switching, i.e., adiabatic propagation, is less than 0.2.degree. according to calculations using the Beam Propagation Method (BPM) made by K. Mitsunaga, K. Murakami, M. Masuda, and J. Koyama in their article entitled, "Optical LiNbO.sub.3 3-branched Waveguide and its Application to a 4-port Optical Switch" appearing in Vol. 19, No. 22 of Applied Optics, Nov. 15, 1980, pp. 3837-3842, (hereafter "Mitsunaga et al."). The beam propagation method (BPM) is a method for calculating the propagation of light through a waveguide when the electric field is localized. BPM is discussed in an article entitled, "Light Propagation in Graded-Index Optical Fibers," by M. D. Feit and J. A. Fleck, Jr., appearing in Applied Optics, Vol. 17, No. 24, Dec. 15, 1978, pp. 3990-3998.
Switches having three outputs (or inputs) are also known. U.S. Pat. No. 4,813,757 (Sakano) discloses a 1.times.3-branch switch having an angle between the center branch and each side branch of 7.degree., which is much too large to allow adiabatic propagation. Mitsunaga et al. disclose a 1.times.3-branch switch having an angle of 1.degree. between adjacent branches, which is also too large to allow adiabatic propagation. In an article entitled, "Design Optimization and Implementation of an Optical Ti:LiNbO.sub.3 3-branch Switch by the Beam Propagation Method" appearing in SHE Vol. 177, Integrated Optics and Optoelectronics, pp. 216-227 (1989), M. A. Serkerka-Bajbus and G. L. Yip disclose a 1.times.3-branch switch having an angle between adjacent branches of 0.01 radians (0.6.degree.), which is still too large to allow substantial adiabatic propagation.
It would be desirable to have a three-branched digital switch which allows for adiabatic propagation with an extinction ratio exceeding 20 dB(100:1) where the length of the switch is shorter than currently available digital switches for improved integration of the switches into optical circuits.